Various data acquisition systems are used in mass spectrometry. One such system used very commonly in inductively coupled plasma mass spectrometry (ICP-MS) is the electron multiplier detector, which detects ions impinging on its dynodes and amplifies the resultant signal to a usable level. Because electron multipliers operating in a pulse count mode have a limited range (too high an ion flux causes saturation), a separate mode of operation of the same detector, namely an analog mode, has commonly been implemented. However, this lacks speed of response and the ability to detect very low and very high signals at the same scan time. Thus, in order to detect these signals simultaneously at high speed, dual output electron multipliers (also called simultaneous mode electron multipliers) were constructed and placed on the market in about 1979. Simultaneous mode electron multipliers contain two dynode stages in series, separated by an analog collector, and also having a protection dynode and a ground dynode. Ions incident on the first dynode stage produce an electron signal, about half of which is collected at the analog collector to produce an analog signal. If the voltage on the protection dynode is set at the appropriate level, the remaining electrons pass through the second stage to produce a pulse count signal. If and when the analog signal rises to a specified level (indicating a relatively high ion flux), the high analog signal triggers application of a suitable voltage to the protection dynode to prevent electrons from passing through the second stage and burning out the detector. Simultaneous mode electron multiplier detectors are marketed by Galileo ElectroOptics Corp. of Sturbridge, Mass., and by ETP Scientific of Auburne, Mass.
The use of simultaneous mode electron multiplier detectors has been described by M. J. Kristo and C. G. Enke in an article in Rev. Sci. Instruments, 59(3), March, 1988. In the system there described, the processing system detects whether the protection dynode is on or off, and if it is on or if there is any doubt about whether it is on, only analog data is acquired.
The prior methods of using simultaneous mode electron multiplier detectors have various disadvantages. One disadvantage is that the user receives no information as to whether the pulse counting or analog signal is being used. Thus, if the gain of the analog stage or pulse stage is drifting, the conversion factor between the two signals may become inaccurate. A second disadvantage is that each point in each peak is constructed using only one signal or the other, but it is not always desirable to use the analog signal converted to correlate with the pulse signal, and prior art systems do not permit a choice. A third disadvantage is that prior art simultaneous mode detectors do not collect and store pulse count, analog and converted analog data so as to permit any or all data to be manipulated later for analysis or diagnostic purposes. A fourth disadvantage of prior art systems is that they do not conveniently allow a process known as "peak hopping", where only peak maxima are measured for a variety of signal levels and elements.